Method for operating a high-pressure discharge lamp with a variable power

ABSTRACT

A method for operating a high-pressure discharge lamp with a variable power is disclosed. Said method utilizing at least the following steps: (a) providing high-pressure discharge lamp, which has a nominal operating power; (b) operating the lamp with an instantaneous power, where the instantaneous power lies within relative lower and upper power limits, respectively, and said limits may depend on an average power, and said instantaneous power may lie within a predetermined absolute lower and upper power limit; (c) determining the average power from a one-sided moving average value of the instantaneous power or the exponential smoothing of the instantaneous power of a time segment of predefined length.

TECHNICAL FIELD

The invention relates to a method for operating a high-pressuredischarge lamp with a variable power, which method can be used e.g. invideo projection systems.

BACKGROUND

The invention relates to a method for operating high-pressure dischargelamps, which method can be used e.g. in video projection systems, andoperates the high-pressure discharge lamp with a variable power in orderto minimize the energy consumption of the system and to increase thecontrast ratio e.g. of the image to be reproduced. Primarily during thereproduction of film material, the image brightness is often very low(e.g. night scenes). In order to increase the dynamic contrast and inorder to save energy and also in order to increase the average lamplifetime by means of operation at on average lower power, therefore, thelamp is in this case intended to be adapted in terms of its powerdynamically with the image content image by image. Although this ispossible in principle, there is a problem here: if the momentarymodulation of the lamp power is too great and the electrode temperatureis thus subject to rapid fluctuations, thermal stresses between core pinand electrode filaments give rise to an “uncoiling” of the electrodefilaments in the direction of the center of the arc. This leads veryrapidly to a reduction of the electrode spacing, which then, as a resultof the current limiting usually provided in the operating devices, hasthe effect that the lamps are operated with an excessively low power andcan then also no longer be operated in the normal operating range.

This considerably reduces the lifetime of the high-pressure dischargelamp. In addition, the image reproduction of the projection system isdisturbed.

One solution is to greatly restrict the permitted modulation range, suchthat the “uncoiling” of the electrode filaments no longer occurs, oroccurs so slowly that it is compensated for by the typical burn-back ofthe electrodes during the operating period.

Alternatively, only a very slow modulation could be permitted, such thatthere are only few momentary changes in power, but then a larger rangeof the modulation depth could be used. However, both solutions greatlyrestrict the contrast ratio obtainable by the modulation of the power,which is highly undesirable.

OBJECT

It is an object of the invention to specify a method for operating ahigh-pressure discharge lamp with a variable power, which method doesnot reduce the lifetime of the high-pressure discharge lamp and at thesame time does not reduce the dynamic range of the high-pressuredischarge lamp.

SUMMARY OF THE INVENTION

The object is achieved by the invention by means of a method foroperating a high-pressure discharge lamp with a variable power, whereinthe high-pressure discharge lamp has a nominal power, and is operatedwith an instantaneous power, wherein the instantaneous power lies withina relative lower and upper power limit, which depends on an averagepower, and lies within a predetermined absolute lower and upper powerlimit, wherein the average power is determined from the average value ofthe instantaneous power. Said average value can preferably be determinedas a one-sided moving average value of the instantaneous power or theexponential smoothing of the instantaneous power of a time segment ofpredefined length. This method can effectively prevent theabove-described “uncoiling” of the electrode filaments, withoutexcessively restricting the dynamic range of the high-pressure dischargelamp.

In one preferred embodiment, the instantaneous power is determined atregular time intervals and the average power is composed of theone-sided moving average value of the last x instantaneous powers. Inanother preferred embodiment, the instantaneous power is likewisedetermined at regular time intervals and the average power is composedof an exponential smoothing of the last x instantaneous powers.

The value x of the last x instantaneous powers of which the averagepower is composed in this case preferably lies in the range of between10<x<600. This measure achieves a sufficient smoothing in conjunctionwith sufficient dynamic range.

In this case, the relative lower and upper power limit is preferablydependent on the average power and on the running voltage U_(B) of thehigh-pressure discharge lamp. This is advantageous in the methodaccording to the invention since the modulation depth can be derivedparticularly well from these parameters, and operation of thehigh-pressure discharge lamp at excessively low or excessively highpower is thus avoided. In this case, preferably, the distance betweenthe relative upper and lower limits is smaller when the running voltageU_(B) is smaller, and larger when the running voltage U_(B) is larger.That is to say that the modulation depth increases when the runningvoltage U_(B) is larger, and decreases when the running voltage U_(B) issmaller. As a result, the growth of the electrode tips can be promotedwhen an electrode spacing is too large, while it is prevented when anelectrode spacing is too small. In this regard it has astonishingly beenfound that a large modulation depth leads to increased tip growth, whilea small modulation depth has no effects on the tip growth.

The relative lower power limit in this case preferably lies between 1%and 30% of the nominal power below the average power of thehigh-pressure discharge lamp and the relative upper power limit in thiscase preferably lies between 1% and 30% of the nominal power above theaverage power of the high-pressure discharge lamp. The absolute lowerpower limit preferably lies between 30% and 80% of the nominal power ofthe high-pressure discharge lamp and the absolute upper power limitpreferably lies between 100% and 130% of the nominal power of thehigh-pressure discharge lamp.

In a further embodiment, the relative lower and upper power limits areat equal distances from the average power. However, it is also possibleto configure the limits independently of one another and to choose alarger value for the distance between the upper power limit and theaverage power. This is advantageous if, in the application, the lightcan additionally also be reduced by suitable additional measures (e.g. ashutter). The brightness of the lamp can then rise more rapidly againfrom a lower level to a brighter level, whereas a reduction of thebrightness can be effected more slowly. When reducing the brightness, itis possible to use the additional measure for an immediate reduction ofthe brightness. The lamp is slowly controlled downward, which can nolonger be perceived, however, as a result of the additional measure.This is not possible the other way round, however: if the brightness hasto be available again immediately, the lamp has to achieve the targetbrightness significantly more rapidly, which can be achieved by virtueof the greater distance between the upper power limit and the averagepower.

In accordance with a further aspect of the invention, the desiredinstantaneous power is predefined by an external control unit, and therelative upper and lower power limits are communicated back to thecontrol unit in the event of each change in power. This enables areliable and efficient communication between the operating device of thehigh-pressure discharge lamp and the control unit (e.g. videoelectronics). In this case, the communication back of the relative upperand lower power limits can be effected by means of a pulse-widthmodulated signal or by means of a digital interface. These two variantscan be implemented safely and efficiently by means of a microcontrollerthat is usually already present. In further methods, the relative upperand lower power limits can also be communicated by means of an analoglevel signal or a frequency signal.

In accordance with a further aspect of the invention, the control unitsynchronizes the desired lamp power with external signals, such as, forexample, an image signal or an audio signal.

Further advantageous developments and configurations of the methodaccording to the invention for operating a high-pressure discharge lampwith a variable power are evident from further dependent claims and fromthe following description.

BRIEF DESCRIPTION OF THE DRAWING(S)

Further advantages, features and details of the invention are evidentwith reference to the following description of exemplary embodiments andwith reference to the drawings, in which identical or functionallyidentical elements are provided with identical reference signs. In thefigures:

FIG. 1 a shows a graph for elucidating the method according to theinvention for operating a high-pressure discharge lamp with a variablepower, said graph depicting the predetermined absolute lower and upperpower limits, and also the average power as a curve and the relativelower and upper power limits, which are dependent on the average power,

FIG. 1 b shows a graph for elucidating the method according to theinvention for operating a high-pressure discharge lamp with a variablepower, said graph depicting the predetermined absolute lower and upperpower limits, and also the average power as a curve, and the relativelower and upper power limits, the dependence of which here is decoupledfrom the average power at times for a lifetime-prolonging measure,

FIG. 2 shows an example of a first embodiment of a method according tothe invention with an absolute lower and upper power limit of 60% to100% and a modulation depth Δ_(a) of 20% and an average power whoseone-sided moving average value is averaged over 10 s,

FIG. 3 shows an example of a second embodiment of a method according tothe invention with an absolute lower and upper power limit of 30% to100% and a modulation depth Δ_(a) of 5% and an average power whoseone-sided moving average value is averaged over 2 s,

FIG. 4 shows a graph with a simple test sequence for the firstembodiment of the method according to the invention,

FIG. 5 shows a graph with the same simple test sequence as in FIG. 4 forthe second embodiment of the method according to the invention,

FIG. 6 shows the graphical representation of the permitted modulationdepth Δ_(a) (the range between relative lower and upper power limits) asa function of the running voltage U_(B) of the high-pressure dischargelamp for a third embodiment of the method according to the invention foroperating a high-pressure discharge lamp.

PREFERRED EMBODIMENT OF THE INVENTION

The following explanations repeatedly use terms which will be brieflyexplained here:

The nominal power of the high-pressure discharge lamp is understood hereto mean the rated power for continuous operation as specified by themanufacturer of the high-pressure discharge lamp. The nominal power of ahigh-pressure discharge lamp for projection purposes can be e.g. 120W,150W or 300W.

Hereinafter, instantaneous power is considered to be the power currentlypresent at the high-pressure discharge lamp. In projection applications,the instantaneous power can be calculated at discrete intervals, that isto say e.g. once per image (also called frame). However, theinstantaneous power can also be calculated continuously.

Hereinafter, average power P_(AV) is considered to be a power which isaveraged over a specific time period. Only real instantaneous powers areaveraged, that is to say that the time period extends into the past. Theaverage power P_(AV) can be calculated as a one-sided moving averagevalue, in the case of which the averaging period proceeding from thecurrent point in time extends into the past. The calculationspecifications concerning the one-sided moving average value can befound e.g. in the German Wikipedia article “Gleitender Mittelwert”[“Moving AverageValue”](http://de.wikipedia.org/wiki/Gleitender_Mittelwert#Einseitiger_gleitender_Mittelwert),retrieved on Apr. 5, 2011.

However, the average power P_(AV) can also be calculated by means of anexponential smoothing, in the case of which a weighted average is formedfrom the last power values and a weighting value. The calculationspecifications concerning exponential smoothing can be found e.g. in theGerman Wikipedia article “Exponentielle Glättung” [“ExponentialSmoothing”], (http://de.wikipedia.org/wiki/Exponentielle_Glättung),retrieved on Apr. 5, 2011.

FIG. 1 a shows a graph for elucidating the method according to theinvention for operating a high-pressure discharge lamp with a variablepower, said graph depicting the predetermined absolute lower power limitP_(Mix) and the predetermined absolute upper power limit P_(Max). Theaverage power P_(AV) is depicted as a curve, and the relative lower andupper power limits, which are dependent on the average power P_(AV), aredepicted as vertical arrows. The range between the lower and upperrelative power limits is also designated as the modulation depth Δ_(a).

In one preferred embodiment, the method according to the invention isused to increase the contrast in video applications by adapting theinstantaneous power of the high-pressure discharge lamp to the currentimage content. For this purpose, an external control unit, e.g. videoelectronics, communicates the desired instantaneous power to theoperating device operating the high-pressure discharge lamp. In thiscase, the communication can be effected by means of a digital interface.However, the communication can also be effected by means of a modulatedsignal input into the operating device. The operating device then setsthe desired power at the high-pressure discharge lamp in the context ofthe instantaneously permitted modulation depth. The modulation depthΔ_(a) for the momentary image-by-image modulation (typically 50 Hz to 60Hz or double that for 3D contents) is restricted to a predeterminedvalue, such that the “uncoiling” of the electrode filaments as describedin the introduction does not occur or occurs only very slowly. In orderthen nevertheless to permit a wider range for the modulation of thepower, an additional parameter is introduced: an average power P_(AV)averaged over a longer time period t. The momentary modulation thenbecomes possible in the permitted range with a predetermined modulationdepth always around this average power P_(AV). In other words, there arean upper and a lower relative power limit around said average powerP_(AV). Upon approaching the upper and lower absolute power limits(P_(Max), P_(Min)), the modulation there remains possible in a rangebetween these fixed limits. The operating device carries out the changein power of the high-pressure discharge lamp, said change being desiredby the control unit, in the context of the currently applicable relativelimits and then communicates the relative upper and lower power limitsback to the control unit for a renewed change in power. In this case,the communication back can likewise be effected by means of a PWM signalor by means of a digital interface.

In this case, the desired lamp power can be synchronized with externalsignals. In particular, the control unit can synchronize the desiredlamp power with an image signal. However, the control unit can likewisesynchronize the desired lamp power from an audio signal.

FIG. 1 b shows a graph for elucidating the method according to theinvention for operating a high-pressure discharge lamp with a variablepower, said graph depicting the predetermined absolute lower and upperpower limits, and depicting the average power P_(AV) as a curve. Thedependence of the relative lower and upper power limits is decoupledhere from the average power at times for a lifetime-prolonging measure,as can be seen in the right-hand part of the graph. The permittedexcursion and its start value, the midpoint of the permitted range, canadditionally be time-dependent. If a lifetime-prolonging measurecurrently present requires lamp operation at nominal power or takeseffect ideally only in that case, then the midpoint m_(f) of thepermitted excursion can be decoupled from the average power P_(AV),increased continuously to P_(Max) and be reduced again to the desiredaverage target power P_(AV) after the end of the measure. This isillustrated in the right-hand region of FIG. 1 b. In this case, theaverage power P_(AV) does not necessarily follow the midpoint m_(f) ofthe permitted excursion.

FIG. 2 shows an example of a first embodiment of a method according tothe invention with an absolute lower power limit P_(Min) of 60% of thenominal power and an absolute upper power limit P_(Max) of 100% of thenominal power and a modulation depth Δ_(a) of the power of 20% of thenominal power. The average power P_(AV) is averaged as a one-sidedmoving average value over 10 s. The maximum range of the instantaneouspower is thus defined by 100% of the nominal power (P_(Max)) to 60% ofthe nominal power (P_(Min)). The momentary maximum image-by-imagemodulation, that is to say the modulation depth Δ_(a), is restricted to20% of the nominal power. The one-sided moving average value of theaverage power P_(AV) is determined as follows: P_(AV)=(P1+P2+P3+P4+ . .. +Pn)/n, where n is chosen such that the averaging takes place over 10s, and the instantaneous powers P1 to Pn are determined in each case foran image. At an image refresh frequency of the frame range of e.g. 60Hz, n=600. A momentary brightness adaptation is thus possible at anytime. With persistent image brightness below the minimum permittedpower, the instantaneous power decreases to the lower permitted powerP_(Min) after approximately 15 s as a result of the algorithm. In thiscase, the average value starts initially at 100%, that is to say atnominal power.

FIG. 3 shows an example of a second embodiment of a method according tothe invention with an absolute lower power limit P_(min) of 30% and anabsolute upper power limit P_(Max) of 100%. The modulation depth Δ_(a)of the average power P_(AV) is 5% in this embodiment. The one-sidedmoving average value is averaged only over 2 s in the second embodiment.The maximum range of the instantaneous power is thus adjustable from100% of the nominal power (P_(Max)) to 30% of the nominal power(P_(Min)). The momentary maximum image-by-image modulation, that is tosay the modulation depth (Δ_(a)), is restricted to only 5%. The averagepower is determined by a one-sided moving average value:P_(AV)=(P1+P2+P3+P4+ . . . +Pn)/n, where n is chosen such that theaveraging takes place over ≈2 s. At an image refresh rate of 60 Hz, n isthen equal to 120. The momentary adaptation is relatively small, butwith persistent image brightness below the minimum permitted powerP_(min) the applied power decreases to the lower permitted power P_(min)after approximately 22 s as a result of the algorithm. The average valuestarts initially at 100%.

FIG. 4 shows a graph with a simple test sequence for the firstembodiment of the method according to the invention. The test sequence51 here consists of a brightness jump from 100% of the image brightnessto 50% of the image brightness and back. The image brightness is plottedover the image numbers. The curve P_(M) is the plotted instantaneouspower of the lamp. The initial power jump from 100% of the nominal powerto 80% of the nominal power on account of the permitted modulation depthΔ_(a) of 20% of the nominal power can readily be seen. The curve P_(AV)designates the average power P_(AV), which follows the instantaneouspower P_(M) only after some time on account of the one-sided movingaverage value calculation. Starting from the jump in the instantaneouspower to 80% of the nominal power, the instantaneous power remains at80% until the average power P_(AV) has reached 90% of the nominal power.

The instantaneous power P_(M) then decreases at the same rate as theaverage power P_(AV), with the difference that it is smaller than theaverage power P_(AV) by the permitted 10%. The instantaneous power P_(M)behaves analogously in the event of the jump in image brightness from60% of the nominal power to 100% of the nominal power of thehigh-pressure discharge lamp. The minimum image brightness of 60% isattained here after approximately 900 frames, corresponding toapproximately 15 s (at 60 frames/s).

FIG. 5 shows a graph with the same simple test sequence 51 as in FIG. 4for the second embodiment of the method according to the invention. Forthe sake of better comparability, the jump in image brightness is thesame as in the previous example in FIG. 4. As a result of the differentparameters concerning the modulation depth Δ_(a) and the one-sidedmoving average value, the instantaneous power P_(M) moves differentlyhere than in the first embodiment. At the start, the instantaneous powerP_(M) decreases by the permitted modulation depth Δ_(a) of 5%. Itremains here briefly and then decreases further with the average powerP_(AV). Since the average power is averaged only over 2 seconds in thesecond embodiment, it decreases at a linear rate. The minimum imagebrightness of 50% is attained here after approximately 1300 frames,corresponding to approximately 22 s (at 60 frames/s).

FIG. 6 shows the graphical representation of the permitted modulationdepth Δ_(a) (the range between relative lower and upper power limits) asa function of the running voltage U_(B) (60V-120V) of the high-pressuredischarge lamp for a third embodiment of the method according to theinvention for operating a high-pressure discharge lamp. The modulationdepth Δ_(a) is analogous to the previous embodiments, but the modulationdepth Δ_(a) is additionally dependent on the running voltage U_(B) ofthe high-pressure discharge lamp. At lower running voltages of e.g. 60V,the modulation depth Δ_(a) is small, e.g. 5%, in order to prevent afurther growing together of the electrodes of the high-pressuredischarge lamp. At high running voltages, the permitted modulation depthΔ_(a) is larger, e.g. 30%, since here a growing together of theelectrodes may even be desirable. The permitted modulation depth can belinear, or alternatively in steps or an arbitrary curve shape.

The advantage of the solution according to the invention is primarilythe large available power range with maximum possible momentarymodulation. As a result, it is possible to achieve a greater energysaving with longer lamp lifetime. Moreover, the operating mode can becombined with a lifetime-prolonging measure by virtue of the fact thatthe modulation depth Δ_(a) can turn out to be higher at higher runningvoltages of the high-pressure discharge lamp. In addition, the manner ofoperation is very flexible. Depending on the requirement of the lamps,the ranges can be set individually. In addition, the modulation depthΔ_(a) can be set depending on the running voltage U_(B). A highmodulation leads to the growing together of the electrodes. This isoccasionally desirable at high running voltages, whereas the electrodesshould not grow together any further at low running voltages. It istherefore appropriate to permit a small modulation depth Δ_(a) at lowrunning voltages and a high modulation depth Δ_(a) at high runningvoltages, which leads to an overall lower running voltage U_(B) duringthe lifetime and can thus prolong the lifetime of the high-pressuredischarge lamp.

LIST OF REFERENCE SIGNS

-   55 Image brightness-   P_(AV) Average power-   P_(Max) Absolute upper power limit-   P_(Min) Absolute lower power limit-   Δa Modulation depth-   m_(f) Midpoint-   Φ_(B) Image brightness-   P_(M) Instantaneous power-   U_(B) Running voltage of the high-pressure discharge lamp

1. A method for operating a high-pressure discharge lamp with a variablepower, wherein the high-pressure discharge lamp has a nominal power, andis operated with an instantaneous power, wherein the instantaneous powerlies within a relative lower and upper power limit, which depends on anaverage power, and lies within a predetermined absolute lower and upperpower limit, wherein the average power is determined from the one-sidedmoving average value of the instantaneous power or the exponentialsmoothing of the instantaneous power of a time segment of predefinedlength,
 2. The method as claimed in claim 1, characterized in that theinstantaneous power is determined at regular time intervals and theaverage power is composed of the one-sided moving average value of thelast x instantaneous powers.
 3. The method as claimed in claim 1,characterized in that the instantaneous power is determined at regulartime intervals and the average power is composed of an exponentialsmoothing of the last x instantaneous powers.
 4. The method as claimedin claim 2, characterized in that x lies in the following range:10<x<600.
 5. The method as claimed in claim 1, characterized in that therelative lower and upper power limit is dependent on the average powerand on the running voltage of the high-pressure discharge lamp.
 6. Themethod as claimed in claim 5, characterized in that the distance betweenthe relative upper and lower limits is smaller when the running voltageis smaller in and larger when the running voltage is largest.
 7. Themethod as claimed in claim 1, characterized in that the relative lowerpower limit lies between 1% and 30% of the nominal power below theaverage power and the relative upper power limit lies between 1% and 30%of the nominal power above the average power.
 8. The method as claimedin claim 1, characterized in that the distance between the relativelower power limit and the average power is of the same magnitude as thedistance between the average power and the relative upper power limit.9. The method as claimed in claim 1, characterized in that the distancebetween the relative lower power limit and the average power (P_(AV)) isof a different magnitude, and in particular smaller, compared with thedistance between the average power (P_(AV)) and the relative upper powerlimit.
 10. The method as claimed in claim 1, characterized in that theabsolute lower power limit lies between 30% and 80% of the nominal powerand the absolute upper power limit lies between 100% and 130% of thenominal power.
 11. The method as claimed in claim 1, characterized inthat the desired instantaneous power is predefined by an externalcontrol unit, and the relative upper and lower power limits arecommunicated back to the control unit in the event of each change inpower.
 12. The method as claimed in claim 11, characterized in that thecommunication back is effected by means of a pulse-width-modulatedsignal, an analog level signal, a frequency or a digital signal.
 13. Themethod as claimed in claim 12, characterized in that the control unitsynchronizes the desired lamp power with external signals.
 14. Themethod as claimed in claim 13, characterized in that the control unitsynchronizes the desired lamp power with an image signal.
 15. The methodas claimed in claim 13, characterized in that the control unitsynchronizes the desired lamp power from an audio signal.
 16. The methodas claimed in claim 3, characterized in that x lies in the followingrange: 10<x<600.